Liquid crystal display device wherein the number of light emitting elements activated differs depending on whether display is performed by the first or second liquid crystal panel

ABSTRACT

A liquid crystal display device has first and second liquid crystal panels ( 101, 102 ) mainly comprising liquid crystal cells ( 107, 108 ), respectively, disposed back to back to enable the visual recognition of the liquid crystal panels ( 101, 102 ). A light guide plate ( 112 ) is disposed between the first liquid crystal panel ( 101 ) and the second liquid crystal panel ( 102 ) with a light source ( 114 ) located adjacent to its one end face ( 112   c ), and a polarization separator ( 110 ) located between the first liquid crystal panel ( 101 ) and the light guide plate ( 112 ). Light emitted from the light guide plate ( 112 ) is split into two beams of polarized light: one is emitted to the first liquid crystal panel ( 101 ), and the other to the second liquid crystal panel ( 102 ) through the light guide plate ( 112 ). This constitution thins down a both-sided display liquid crystal display device and reduces power consumption.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device, andparticularly to a liquid crystal display device for both-sided display,having two liquid crystal panels disposed back to back to enable thevisual recognition of the liquid crystal panels.

BACKGROUND TECHNOLOGY

A liquid crystal display device has made rapid progress as a low power,thin and light-weighed display device in various fields. Particularly inrecent years, a portable phone (cellular phone) also has made drasticprogress and all types of portable phones employ the liquid crystaldisplay device.

The liquid crystal display device employed in the portable phone is inmany cases a reflective liquid crystal display device that is designedespecially from the view point of low power consumption so that the lifetime of a battery is elongated. Furthermore, almost all of thereflective liquid crystal display devices are a transflective liquidcrystal display device incorporating a backlight therein. The reason forthis is that a liquid crystal display device is not a light-emittingdisplay device and therefore, when the device is used under conditionswith insufficient light intensity, i. e., under dark conditions, animage to be displayed on the device becomes difficult to view, thereby abacklight illumination is adopted to improve the visibility of an imageto be displayed.

Additionally, as information communication using, for example e-mail,makes progress, a liquid crystal display screen also becomes large insize and accordingly, a portable phone becomes large in size. To addresssuch problem, a portable phone has been developed which advantageouslyallows protection of the liquid crystal display screen and increase inportability.

A user of the portable phone usually carries the phone in a folded formand accordingly, every time when the user wants to know the current timeor get information from display of incoming call, etc., the user needsto unfold the phone. To eliminate such inconvenience, a recent portablephone tends to be constructed so that a second liquid crystal panel isdisposed independently of a first liquid crystal panel in a visibleposition when the portable phone is being folded, on which various typesof information is always displayed.

In this case, since the first liquid crystal panel used when theportable phone is being unfolded and the second liquid crystal panelused when the portable phone is being folded both need to be used evenunder dark conditions, a transflective liquid crystal panelincorporating a backlight therein has been used.

FIG. 13 illustrates the exemplary configuration of a portable phone ofthe type conventionally used. The portable phone 1000 is constructedsuch that a main unit 1100 and a display unit 1200 are coupled togetherby a hinge 1300 so that the main unit 1100 and display unit 1200 arerotatable around the hinge 1300 and openable and closable relative toeach other. Furthermore, the portable phone includes a key board 1109provided on an upper surface of the main unit 1100 and is furtherconstructed the display unit for a both-sided display such that a firstliquid crystal display device 1101 and a second liquid crystal displaydevice 1102 are disposed back to back in the display unit 1200.

The first liquid crystal display device 1101 comprises a first liquidcrystal panel 1103 and a first backlight 1104, and the second liquidcrystal display device 1102 comprises a second liquid crystal panel 1105and a second backlight 1106. Moreover, a first windshield glass 1107 anda second windshield glass 1108 are provided on an internal (right sidein the figure) surface and an external (left side in the figure) surfaceof a casing of the display unit 1200 to allow a user to view the liquidcrystal panels 1103, 1105, respectively.

How the portable phone 1000 shown in FIG. 13 displays images will beexplained below. When a user uses this portable phone 1000, the displayunit 1200 folded toward the main unit 1100 is rotated in a direction ofan arrow in the figure to open the portable phone. In this case, theliquid crystal panel 1103 of the first liquid crystal display device1101 displays an image and the first backlight 1104 is turned on. Atthis point, the liquid crystal display panel 1105 of the second liquidcrystal display device 1102 displays an image, but the second backlight1106 is turned off. Therefore, the user operates the keyboard 1109 ofthe main unit 1100 while viewing display on the first liquid crystaldisplay device 1101.

Thereafter, when the user of the portable phone 1000 uses it in a foldedform, the display unit 1200 is folded by being rotated in the directionopposite the arrow to overlie the main unit 1100 and simultaneously, thedisplay on the liquid crystal panel 1103 of the first liquid crystaldisplay device 1101 is terminated, and the first backlight 1104 also isturned off. Then, the second backlight 1106 is turned on while thedisplay on the liquid crystal panel 1105 of the second liquid crystaldisplay device 1102 is continued. Note that the second backlight 1106 isturned off after a few tens seconds. Moreover, the second backlight 1106is turned on, for example, when an incoming call is received or keypadsare pressed by the user.

However, a problem arises in that two liquid crystal display devices areincorporated in the above-described conventional portable phone, causingincrease in power consumption. Furthermore, each of the two liquidcrystal display devices utilizes only the polarized component of lightfrom a backlight, i.e., only the light transmitting through thepolarizer of each of the display devices, meaning that the lightutilization ratio of liquid crystal display device is 50% at themaximum. Since such a backlight is incorporated within each of theliquid crystal display devices, 50% of light from the backlight isabsorbed, in other words, consumed away by the polarizer of each of theliquid crystal display devices. That is, to achieve desired intensity oflight, the portable phone as described above dissipates twice the poweras a portable phone with single liquid crystal display device.

Additionally, since the portable phone is configured to dispose twoliquid crystal display devices so that the rear surfaces of the displaydevices face each other, the thickness of the display unit is increased,disadvantageously decreasing portability of the phone. For example, incase of the portable phone 1000 shown in FIG. 13, the liquid crystalpanel 1103 of the first liquid crystal display device 1101 would have athickness of 1.5 mm and the first backlight 1104 would have a thicknessof 1 mm, meaning that the those two components would have a thicknesstotaling 2.5 mm. Since the second liquid crystal display device 1102 hasthe same structure as the first liquid crystal display device 1101, thefirst and second liquid crystal display devices have approximately thesame thickness and when the two liquid crystal display devices areassembled in superimposed relation with back to back each other, thethickness of those display devices unfavorably becomes equal to about 5mm. Additionally, when other necessary components such as a supportframe are mounted to those display devices, the portable phone becomesvery thick.

As described above, the conventional liquid crystal display devicecapable of displaying images on both sides includes some problems. Thatis, 50% or more of light from the backlight is consumed away andtherefore, the portable phone disadvantageously dissipates twice thepower as a portable phone with a single display, and further, thedisplay unit of electronic equipments incorporating therein suchconventional liquid crystal display device becomes thick, decreasing theportability of electronic equipments.

DISCLOSURE OF THE INVENTION

The present invention has been conceived to solve the above-describedproblems and an aspect of the invention is to thin a liquid crystaldisplay device, which is capable of displaying images on both sides, andincrease the portability of electronic equipments that incorporatetherein the display device, and further, increase the utilization ratioof light from a backlight to allow reduction in power consumption of theelectronic equipments.

Accordingly, the present invention provides a liquid crystal displaydevice constructed such that first and second liquid crystal panels,each consisting mainly of liquid crystal cells each of which has aliquid crystal layer sandwiched between two transparent substrates, aredisposed back to back each other to enable the visual recognition of thefirst and second liquid crystal panels. The liquid crystal displaydevice is constructed as follows.

That is, a light guide plate is disposed between the first liquidcrystal panel and second liquid crystal panel, a light source isdisposed adjacent at least one end surface of the light guide plate, anda polarization separator is disposed between the first liquid crystalpanel and the light guide plate.

Furthermore, light emitted from the light guide plate is divided intotwo polarized lights by the polarization separator and one of thepolarized lights is emitted toward the first liquid crystal panel, andthe other of the polarized lights is emitted toward the second liquidcrystal panel via the light guide plate.

Additionally, the liquid crystal display device may preferably beconfigured so that when the first liquid crystal panel and second liquidcrystal panel each have polarizers on both sides of the liquid crystalcell, the polarization separator having a polarization transmission axisfor transmitting one of linearly polarized lights whose polarizingdirections are orthogonal to each other and a polarization reflectionaxis for reflecting the other of linearly polarized lights is disposedso that the polarization transmission axis is closely aligned with thepolarization transmission axis of the polarizer, facing the polarizationseparator, of the first liquid crystal panel and the polarizationreflection axis is closely aligned with the polarization transmissionaxis of the polarizer, facing the light guide plate, of the secondliquid crystal panel.

Furthermore, the liquid crystal display device may also be configured sothat a transfective reflector having no polarization capability isdisposed between the second liquid crystal panel and the light guideplate, light emitted from the light guide plate is divided into twopolarized lights by the polarization separator, one of the polarizedlights is emitted toward the first liquid crystal panel, and the otherof the polarized lights is emitted toward said second liquid crystalpanel via the light guide plate and transflective reflector.

In such a case, the polarization separator may be disposed so that thereflection axis thereof is closely aligned with the polarizationtransmission axis of the polarizer, facing the transflective reflector,of the second liquid crystal panel.

The liquid crystal display device having the above configuration may beconfigured so that the light source is comprised of a plurality of lightemitting elements and the number of light emitting elements activated atthe time of display on the first liquid crystal panel and at the time ofdisplay on the second liquid crystal panel is made different from eachother. In this case, it is preferred that a display area of the secondliquid crystal panel is smaller than that of the first liquid crystalpanel.

Still furthermore, the liquid crystal display device may be configuredso that when display is performed by the first liquid crystal panel, allof the plurality of light emitting elements are allowed to emit lightsand when display is performed by the second liquid crystal panel, only aspecific number of the plurality of light emitting elements are allowedto emit lights to sufficiently illuminate the display area of the secondliquid crystal panel.

Moreover, the liquid crystal display device may also be configured sothat a first polarization separator is disposed between the first liquidcrystal panel and the light guide plate, a second polarization separatoris disposed between the second liquid crystal panel and the light guideplate, light emitted from the light guide plate is divided into twopolarized lights by each of the first and second polarizationseparators, so that one of the polarized lights is emitted toward theside of the first liquid crystal panel and the other of the polarizedlights is emitted toward the side of the second liquid crystal panel,the light source is comprised of a plurality of light emitting elements,and the number of light emitting elements activated at the time ofdisplay on the first liquid crystal panel and the number of lightemitting elements activated at the time of display on the second liquidcrystal panel are made different from each other.

In this case, it is preferred that the first polarization separator andsecond polarization separator have their polarization transmission axesorthogonal to each other, the polarization transmission axis of thefirst polarization separator is closely aligned with the polarizationtransmission axis of the polarizer, facing the first polarizationseparator, of the first liquid crystal panel, and the polarizationtransmission axis of the second polarization separator is closelyaligned with the polarization transmission axis of the polarizer, facingthe second polarization separator, of the second liquid crystal panel.

Furthermore, it is preferred that the display area of the second liquidcrystal panel is made smaller than the display area of the first liquidcrystal panel and when display is performed by the first liquid crystalpanel, all of the plurality of light emitting elements are allowed toemit lights, and when display is performed by the second liquid crystalpanel, only a specific number of the plurality of light emittingelements are allowed to emit lights to sufficiently illuminate thedisplay area of the second liquid crystal panel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross sectional view illustrating a firstembodiment of a liquid crystal display device according to theinvention;

FIG. 2 is an enlarged partial perspective view illustrating a lightsource and a light guide plate of FIG. 1;

FIG. 3 is an explanatory diagram illustrating a positional relationshipbetween the individual polarization axes of second, third polarizers andpolarization separator of FIG. 1;

FIG. 4 is an explanatory diagram illustrating a schematic cross sectionto explain how the liquid crystal display device of FIG. 1 performs adisplay function;

FIG. 5 is a schematic cross sectional view illustrating a secondembodiment of the liquid crystal display device according to theinvention;

FIG. 6 is a schematic cross sectional view to explain how the liquidcrystal display device of FIG. 5 performs a display function;

FIG. 7 is a side view illustrating only the light guide plate and lightsource used in a third embodiment of the liquid crystal display deviceaccording to the invention;

FIG. 8 is a schematic cross sectional view to explain how the liquidcrystal display device of the third embodiment according to theinvention performs a display function;

FIG. 9 is a perspective view of the general configuration of a fourthembodiment of the liquid crystal display device according to theinvention, omitting illustration of a polarization separator and/or atransflective reflector;

FIG. 10 is a schematic cross sectional view, similar to that of FIG. 8,to explain how the liquid crystal display device of a fifth embodimentaccording to the invention performs a display function;

FIG. 11 is a perspective view of the general configuration of the liquidcrystal display device of the fifth embodiment, omitting illustration oftwo polarization separators;

FIG. 12 is an explanatory diagram illustrating a positional relationshipbetween the individual polarization axes of second, third polarizers andfirst, second polarization separators of FIG. 10; and

FIG. 13 is a schematic cross sectional view illustrating the exemplaryconfiguration of a portable phone incorporating therein a conventionalliquid crystal display device for both-sided display.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will beexplained in detail below with reference to the accompanying drawings.

First Embodiment: FIGS. 1 to 4

FIG. 1 is a schematic cross sectional view illustrating a firstembodiment of a liquid crystal display device according to theinvention.

The liquid crystal display device shown in FIG. 1 has a first liquidcrystal panel 101, polarization separator 110, light guide plate 112,and a second liquid crystal panel 102 arranged in this order from top tobottom in the figure. Accordingly, in this embodiment, the polarizationseparator 110 is provided only between the first liquid crystal panel101 and the light guide plate 112, and no component is provided betweenthe second liquid crystal panel 102 and the light guide plate 112.

The first liquid crystal panel 101 includes a first liquid crystal cell107 having a liquid crystal layer sandwiched between two glasssubstrates, a first polarizer 103 disposed on a viewing side “A”, and asecond polarizer 105 disposed on a side opposite the viewing side “A”.Furthermore, though not shown, the first liquid crystal cell 107 has atransflective layer provided therein for transmitting a part of lightincident thereon and reflecting the remaining part of the light, therebyconstituting a transflective liquid crystal cell.

Furthermore, the second liquid crystal panel 102 includes a secondliquid crystal cell 108 having a liquid crystal layer sandwiched betweentwo glass substrates, a third polarizer 104 disposed on the side of thelight guide plate 112, and a fourth polarizer 106 disposed on a viewingside “B”. Moreover, a light source 114 is disposed adjacent one endsurface 112 c of the light guide plate 112.

Subsequently, individual components of the device will be explained indetail. The first to fourth polarizers are implemented by anabsorption-type polarizer. The absorption-type polarizer is a typicalpolarizer prepared by staining a stretched film with iodine or dichroicdye and has a polarization transmission axis and an absorption axisorthogonal to each other, and transmits light whose polarizing directionis parallel to the polarization transmission axis and absorbs lightwhose polarizing direction is parallel to the absorption axis.

The first liquid crystal panel 101 is configured so that thepolarization transmission axes of the first polarizer 103 and secondpolarizer 105 are adjusted to allow the panel to perform white displaywhen no voltage is applied to the first liquid crystal cell 107 and thenthose polarizers are attached to the first liquid crystal cell 107. Thatis, the first liquid crystal panel 101 is configured to operate in thenormally white mode. Moreover, the first liquid crystal panel 101 is atranflective liquid crystal panel having a transflective layer providedwithin the first liquid crystal cell 107.

Likewise, the second liquid crystal panel 102 is configured so that thepolarization transmission axes of the third polarizer 104 and fourthpolarizer 106 are adjusted to allow the panel to perform white displaywhen no voltage is applied to the second liquid crystal cell 108 andthen those polarizers are attached to the second liquid crystal cell108. That is, the second liquid crystal panel 102 is configured tooperate in the normally white mode.

Thereafter, the polarization separator 110 disposed between the firstliquid crystal panel 101 and second liquid crystal panel 102 will beexplained. The polarization separator 110 serves to divide lightincident thereon into two polarized components. Examples of suchpolarization separators include a polarization separator configured sothat a circularly polarized component is split by a cholestric liquidcrystal into a right-handed circularly polarized component and aleft-handed circularly polarized component and then those right-handedand left-handed circularly polarized components each are converted to alinearly polarized component by a ¼ wavelength plate, and a polarizationseparator configured so that thin films with refractive indexesdifferent from one another are laminated to form a polarizationseparator for dividing a linearly polarized component into two linearlypolarized components orthogonal to each other. The embodiment employsthe latter polarization separator. The polarization separator 110 has apolarization transmission axis for transmitting one of linearlypolarized components of incident light whose polarizing directions areorthogonal to each other and a reflection axis for reflecting the otherof linearly polarized components.

Then, how the second polarizer 105, the polarization separator 110 andthe third polarizer 104, all of which are critical components forimplementing the invention, are optically arranged will be explained indetail.

FIG. 3 illustrates a positional relationship between the polarizationaxes of the individual polarizers and the polarization separator. InFIG. 3, individual arrows indicate the corresponding polarization axesand a solid line represents a polarization transmission axis, and abroken line represents a polarization reflection axis.

As previously described, a polarization transmission axis 301 and apolarization reflection axis 302 of the polarization separator 110 areorthogonal to each other. A polarization transmission axis 303 of thesecond polarizer 105 is aligned parallel to the polarizationtransmission axis 301 of the polarization separator 110. A polarizationtransmission axis 304 of the third polarizer 104 is aligned parallel tothe polarization reflection axis 302 of the polarization separator 110.In this case, the polarization transmission axis 303 of the secondpolarizer 105 and the polarization transmission axis 304 of the thirdpolarizer 104 are aligned orthogonal to each other.

The second polarizer 105, third polarizer 104 and polarization separator110 are arranged as described above. In this case, the first polarizer103 and fourth polarizer 106 are arranged to allow the individual liquidcrystal panels 101, 102 to operate in the normally white mode.

Subsequently, the light guide plate 112 will be explained with referenceto FIG. 2. The light guide plate 112 is made of a non-colored andtransparent acrylic material having a thickness of 0.7 mm. A frontsurface 112 a is planar and prisms 112 b are formed on a rear surface,and the light guide plate operates so that light totally reflected bythe front surface 112 a and guided within the plate is reflected by theprism 112 b and exits the front surface 112 a. The prisms 112 b areformed in the shape of triangular mountains having a height of 20 μm andarranged at a pitch of 300 μm.

Referring to FIG. 2, an upwardly inclined plane beginning with the sideof the light source 114 and inclined at an angle α of 4.6° is formedextending a distance of L1=245 μm, and a downwardly inclined planebeginning with the point at which the upwardly inclined plane isterminated and inclined at an angle β of 20° is formed extending adistance of L2=55 μm. In this case, the prism 112 b has a height H of 20μm. The prism 112 b is repeated at a pitch of 0.3 mm in the longitudinaldirection of the light guide plate 112. Those prisms are formed using aninjection molding process to ensure that occurrence of opticaldistortion is avoided as possible.

The light source 114 is disposed so as to be close to one (positioned onthe side of a lower portion of the longer inclined plane of the prism112 b) end surface 112 c of the light guide plate 112 in thelongitudinal direction thereof. In the embodiment, a white LED array isemployed as the light source 114. The light source 114 is not limitedthereto, but may be implemented by a cold-cathode tube. Any linear lightsource may be employed which is capable of emitting light with arelatively uniform distribution of illumination intensity to the one endsurface 112 c of the light guide plate 112.

Subsequently, how the liquid crystal display device performs a displayfunction will be explained with reference to FIG. 4 as an explanatorydiagram illustrating a schematic cross section of the device.

In FIG. 4, a light flux 401 represents a portion of light fluxes emittedfrom the light source 114 and its travel path. A light flux 402represents a portion of the light flux 401 and the relative amount oflight flux transmitting through the polarization separator 110, and itstravel path. A light flux 403 represents a portion of the light flux 401and the relative amount of light flux reflected by the polarizationseparator 110, and its travel path. Light fluxes 406, 404 represent aportion of external lights incoming from a viewing side “A” and viewingside “B”, respectively. A light flux 407 represents a portion of thelight flux 406 and the relative amount of light flux reflected by thetransflective layer within the first liquid crystal panel 101 andexiting to the viewing side “A”, and its travel path. A light flux 408represents a portion of the light flux 406 and the relative amount oflight flux transmitting through the transflective layer within the firstliquid crystal panel 101 and exiting to the side of the polarizationseparator 110, and its travel path. A light flux 405 represents aportion of the light flux 404 and the relative amount of light fluxtransmitting through the second liquid crystal panel 102 and its travelpath.

It should be noted that arrows, representing the individual lightfluxes, of FIG. 4 mean that an arrow with dots indicates a non-polarizedlight, a white arrow indicates only a linearly polarized light componentvertical to the paper, and a black arrow indicates only a linearlypolarized light component parallel to the paper.

First, how a display function is performed when the light source 114 isturned on will be explained. The light flux 401 emitted from the lightsource 114 enters the light guide plate 112 and is guided within thelight guide plate 112 while being totally reflected. When the light fluxis guided along a light path and impinges on the short side of theprism, the light flux totally reflected by the short side exits to theside of the first liquid crystal panel 101. The light flux 401 is alight flux maintaining its original non-polarized state of when it isemitted from the light source 114. The light flux 401 exiting the lightguide plate 112 enters the polarization separator 110 and the light flux402 having a polarized component parallel to the polarizationtransmission axis 301 of the polarization separator 110 transmitsthrough the separator 110. On the other hand, the light flux 403 havinga polarized component parallel to the polarization reflection axis 302of the polarization separator 110 is reflected by the separator andagain enters the light guide plate 112.

Since the polarization transmission axis 303 of the second polarizer 105is parallel to the polarization transmission axis 301 of thepolarization separator 110 as shown in FIG. 3, the light flux 402transmits through the second polarizer 105. Furthermore, a portion ofthe light flux 402 transmits through the transflective layer formedwithin the liquid crystal cell 107. As previously explained, the firstliquid crystal panel 101 is configured to operate in the normally whitemode and therefore the light flux 402 exits to the viewing side “A”without experiencing a change.

At this point, since an observer on the viewing side “A” views the lightflux 402, the liquid crystal display device is able to perform imagedisplay by using the light flux 402 as a light source and controlling avoltage applied to the first liquid crystal panel 101.

On the other hand, the light flux 403 reflected by the polarizationseparator 110 again enters the light guide plate 112 and exits the lightguide plate 112 without neither being absorbed, reflected nor scatteredwhile maintaining its linearly polarized state because the light guideplate 112 is made of a thin acrylic plate with high transparency.Thereafter, the light flux 403 enters the polarizer 104. As can be seenfrom FIG. 3, the polarization transmission axis 304 of the thirdpolarizer 104 is parallel to the polarization reflection axis 302 of thepolarization separator 110 and therefore the light flux 403 alsotransmits through the polarizer 104. Note that since the second liquidcrystal panel 102 is also configured to operate in the normally whitemode, the light flux 403 exits to the viewing side “B” withoutexperiencing a change.

At this point, when an observer on the viewing side “B” views the lightflux 402, the liquid crystal display device is able to perform imagedisplay by using the light flux 403 as a light source and controlling avoltage applied to the second liquid crystal panel 102.

Subsequently, how a display function is performed when the light source114 is not turned on will be explained. The light flux 404 as externallight incident from the viewing side “B” travels so that only apolarized component parallel to the polarization transmission axis 304of the fourth polarizer 106 transmits through the polarizer 106 and alsotransmits through the third polarizer 104 because the second liquidcrystal panel operates in the normally white mode, and enters the lightguide plate 112. Since the light guide plate 112 is a transparentsubstrate, the above-stated polarized component of the light flux 404transmits through the plate while maintaining its linearly polarizedstate and enters the polarization separator 110. At this point, since adirection in which the light flux 405 is linearly polarized coincideswith the polarization reflection axis 302 of the polarization separator110, the light flux 405 is reflected by the polarization separator 110and again transmits through the light guide plate 112 and second liquidcrystal panel 102, and exits to the viewing side “B”.

Thus, at this point, an observer on the viewing side “B” is able to viewexternal light incident from the viewing side “B” and then reflected bythe separator, and therefore, the liquid crystal display device is ableto perform image display by controlling a voltage applied to the secondliquid crystal panel 102.

The light flux 406 as external light incident from the viewing side “A”travels so that only a polarized component parallel to the polarizationtransmission axis 303 of the first polarizer 103 transmits through thepolarizer 103 and is reflected by the transflective layer formed withinthe liquid crystal cell 107, thereby forming a light flux 407, and againtransmits through the first polarizer 103 and exits to the viewing side“A”. Note that a light flux 408, a part of the light flux 406, transmitsthrough the transflective layer formed within the liquid crystal cell107 and has its polarized direction rotated 90 degrees by the liquidcrystal layer, and also transmits through the second polarizer 105.

Thus, at this point, an observer on the viewing side “A” is able to viewexternal light incident from the viewing side “A” and then reflected bythe transflective layer, and therefore, the liquid crystal displaydevice is able to perform image display by controlling a voltage appliedto the first liquid crystal panel 101.

As described above, the liquid crystal display device of the embodimentis able to perform image display by using any one of light from thelight source 114 and external lights from the viewing sides “A” and “B”.In this case, it should be understood that as shown in FIG. 4, the lightflux 401 from the light source 114 is divided into two polarized lightcomponents by the polarization separator 110 and the individualpolarized light components serve as illumination light withoutexperiencing loss to illuminate the first and second liquid crystalpanels. This allows the light source 114 to illuminate the two liquidcrystal panels disposed on both sides of the device. Furthermore, theamounts of lights utilized by the two liquid crystal panels 101, 102each are 50% of the amount of light from the light source 114, meaningthat the liquid crystal display device of the embodiment is capable ofutilizing 100% of the amount of light from the light source 114.

Moreover, when the device performs display function using externallights, an observer views the liquid crystal display device of theembodiment from the viewing side “A”, how light travels after itsentrance into the liquid crystal panel is completely the same as wouldbe observed in a conventional tranflective liquid crystal panel, therebyallowing the observer to recognize an image to be displayed on the panelin the same fashion. When an observer views the device of the embodimentfrom the viewing side “B,” the liquid crystal panel of the embodiment isable to provide brighter display than the conventional tranflectiveliquid crystal panel. The reason for this is that the conventionaltranflective liquid crystal panel has a semi-transmissive plate providedtherein, however the liquid crystal panel of the embodiment does notinclude a semi-transmissive plate and therefore about half of theexternal light flux 404 enters the panel, and is totally reflected bythe polarization separator 110 and returns back to the viewing side “B”.Accordingly, light reflectance upon reflection of light becomes high,allowing the liquid crystal panel of the embodiment to provide brighterdisplay. Also upon transmission of light, since a semi-transmissiveplate is not present within the panel, brighter transmission light thanwould be in the conventional tranflective liquid crystal panel can beobtained.

Additionally, although the embodiment employs a tranflective liquidcrystal panel as the first liquid crystal panel 101, instead, it mayemploy a transmissive liquid crystal panel. As long as a relationshipbetween the polarized components divided by the polarization separator110 and the third polarizer 105 and fourth polarizer 104 is satisfied,any type of liquid crystal panel can be employed in the liquid crystaldisplay device of the invention. Moreover, it is also apparent thatirrespective of passive matrix drive or active matrix drive, any type ofdrive method for liquid crystal panel can be applied to the liquidcrystal panel of the embodiment. Furthermore, although explanation hasbeen made of the normally white mode as a display mode, the normallyblack mode may also be applied to the embodiment.

Second Embodiment: FIGS. 5 and 6

Thereafter, a second embodiment of the liquid crystal display deviceaccording to the invention will be explained. FIG. 5 is a schematiccross sectional view of the liquid crystal display device and FIG. 6 isa schematic cross sectional view to explain how the liquid crystaldisplay device performs a display function.

The second embodiment is different from the first embodiment in that atransflective reflector 120 is disposed between a light guide plate 112and a liquid crystal panel 102, and a transflective layer is notprovided within a first liquid crystal panel 502. The configurationother than the above-mentioned configuration and display theory areapproximately the same as those explained in the description of thefirst embodiment. The second embodiment will be explained in detailbelow.

Referring to FIG. 5, the transflective reflector 120 is disposed betweenthe light guide plate 112 and the second liquid crystal panel 102. Thetransflective reflector 120 is an optical element like a half mirrorthat has no polarizing capability and serves to transmit a portion oflight incident thereon and reflect the remainder of the light. Forexample, 20 to 40% of incident light is allowed to transmit and 60 to80% of incident light is reflected.

Furthermore, a first liquid crystal cell 503 constituting the firstliquid crystal panel 502 is a transmissive liquid crystal cell having notransflective layer. The configuration other than the above-statedconfiguration is the same as that shown in FIG. 1 and therefore detailedexplanation thereof is omitted.

A positional relationship, employed in the liquid crystal displaydevice, between individual polarization axes of a second polarizer 105,polarization separator 110 and third polarizer 104 is the same as thatexplained in the description of the first embodiment shown in FIG. 3.Moreover, the individual polarizers 103, 105, 104 and 106 are disposedso as to allow the first and second liquid crystal panels 502, 102 tooperate in the normally white mode.

Subsequently, how the liquid crystal display device performs a displayfunction will be explained with reference to FIG. 6. Arrows,representing individual light fluxes, of FIG. 6 also mean that an arrowwith dots indicates a non-polarized light, a white arrow indicates onlya linearly polarized light component vertical to the paper, and a blackarrow indicates only a linearly polarized light component parallel tothe paper.

In FIG. 6, a light flux 701 represents the relative amount of light fluxtransmitting through the first liquid crystal panel 502 and thepolarization separator 110 and its travel path when the light flux 406as external light incident from the viewing side “A” enters the panel502. A light flux 702 represents a light flux after the light flux 701is reflected by the polarization separator 110 and the relative amountof light flux reflected and then exiting to the viewing side “A”, andits travel path. A light flux 709 represents the relative amount oflight flux transmitting through the transflective reflector 120 and itstravel path.

A light flux 703 is produced so that a light flux from a light source114 enters the light guide plate 112, is totally reflected by the plate,reaches the polarization separator 110, and is reflected by theseparator. A light flux 704 represents a portion of the light flux 703and the relative amount of light flux transmitting through thetransflective reflector 120, and its travel path. A light flux 705represents a portion of the light flux 703 and the relative amount oflight flux reflected by the transflective reflector 120, and its travelpath.

A light flux 706 represents the relative amount of light flux that isproduced after a portion of an external light flux 404 incident from theviewing side “B” transmits through the second liquid crystal panel 102.A light flux 707 represents the relative amount of light flux that isproduced after a portion of the light flux 706 is reflected by thetransflective reflector 120 and then exits to the viewing side “B”, andits travel path. A light flux 708 represents the relative amount oflight flux that is produced after a portion of the light flux 706transmits through the transflective reflector 120, and its travel path.

A light flux 700, a portion of the light flux 401, having a linearlypolarized light component parallel to the polarization transmission axis301 (refer to FIG. 3) of the polarization separator 110 transmitsthrough it and also through the second polarizer 105. Then, the lightflux 700 exits to the viewing side “A” without reducing its magnitudebecause no transflective layer is present within the first liquidcrystal cell 503 in the present embodiment. On the other hand, the lightflux 703 having a linearly polarized light component parallel to thepolarization reflection axis 302 (refer to FIG. 3) of the polarizationseparator 110 is reflected by it and again transmits through the lightguide plate 112, and enters the transflective reflector 120. At thispoint, a portion of the linearly polarized component of the light flux703 transmits through the transflective reflector 120, producing thelight flux 704 which enters the third polarizer 104.

As can be seen from FIG. 3, the polarization transmission axis 304 ofthe third polarizer 104 and the polarization reflection axis 302 of thepolarization separator 110 are parallel to each other and therefore thelight flux 704 transmits through the polarizer 104. Note that since thesecond liquid crystal panel 102 operates in the normally white mode, thelight flux 704 exits to the viewing side “B” without experiencing achange. The light flux 705 reflected by the transflective reflector 120also is nearly totally reflected by the polarization separator 110 andis again returned back to the transflective reflector 120, and a portionof the returned light flux transmits through the reflector and theremainder of the returned light flux is reflected by the reflector, andthe aforementioned operation is repeated, allowing almost all amount oflight flux 705 to exit to the viewing side “B.” Accordingly, at thispoint, the liquid crystal display device is able to perform imagedisplay by using light emitted from the light source 114 and controllinga voltage applied to the second liquid crystal panel 102.

The light flux 404 incident as external light from the viewing side “B”travels so that only the component, polarized by the fourth polarizer106, of the light flux transmits through the polarizer 106, producing alight flux 706, and the light flux 706 transmits also through the thirdpolarizer 104 because the second liquid crystal panel operates in thenormally white mode and enters the transflective reflector 120. At thispoint, a light flux 708, a portion of the light flux 706, transmitsthrough the transflective reflector 120, but almost all of the lightflux 706 is reflected by the reflector, producing a light flux 707, andthe light flux 707 again enters the second liquid crystal panel 102 andexits to the viewing side “B.”

Accordingly, at this point, an observer on the viewing side “B” viewsthe reflected component of external light incident from the viewing side“B” and therefore the liquid crystal display device is able to performimage display by controlling a voltage applied to the second liquidcrystal panel 102.

The light flux 406 incident as external light from the viewing side “A”transmits through the first polarizer 103, producing the light flux 701,and the light flux 701 transmits also through the second polarizer 105because the first liquid crystal panel operates in the normally whitemode and enters the polarization separator 110. At this point, thepolarization transmission axis 301 (refer to FIG. 3) of the polarizationseparator 110 and the polarization direction of the light flux 701 areparallel to each other, and therefore, the light flux 701 transmits alsothrough the polarization separator 110 and enters the light guide plate112. Since the light guide plate 112 is a transparent substrate, thelight flux 701 transmits through the plate while maintaining itslinearly polarized state and enters the transflective reflector 120.

At this point, a portion of the light flux 701 transmits through thetransflective reflector 120, producing the light flux 709, but since thelight flux 709 is linearly polarized light parallel to the absorptionaxis of the third polarizer 104 of the second liquid crystal panel 102,almost all of the light flux 709 is absorbed by the polarizer 104. Then,almost all of the light flux 701 is reflected by the transflectivereflector 120, producing a light flux 702, and the light flux 702 againtransmits through the polarization separator 110 and the first liquidcrystal panel 502, and exits to the viewing side “A.” Therefore, anobserver on the viewing side “A” views the reflected component ofexternal light incident from the viewing side “A” and accordingly, theliquid crystal display device is able to perform image display bycontrolling a voltage applied to the first liquid crystal panel 502.

As described above, in the present embodiment, the liquid crystaldisplay device is able to perform image display by using any one oflight from the light source 114 and the external light flux 406 from theviewing side “A” and the external light flux 404 from the viewing sides“B.” In this case, it should be understood that as shown in FIG. 6, thelight flux 401 from the light source 114 is divided into two polarizedcomponents by the polarization separator 110 and the individualpolarized components serve as a light source without experiencing lossto illuminate the first and second liquid crystal panels 502, 102. Thisallows the light source 114 to illuminate the two liquid crystal panelsdisposed on both sides of the device.

When the device performs a display function using external light, anobserver on the viewing side “A” views an image on the panel utilizing alight flux reflected by the transflective reflector 120 disposed withthe interposition of the light guide plate 112, which operation isdifferent from that performed when the conventional tranflective liquidcrystal panel is used. Accordingly, the liquid crystal display device ofthe embodiment allows display with high reflectance and brightness uponreflection of light.

Furthermore, instead of the transflective reflector 120 and polarizer104, a slightly transmissive absorber and reflection-type polarizer maybe disposed. For example, TDF (trademark) available from 3M company hasa transrucent absorber and reflection-type polarizer integrated thereinand therefore, instead of the transflective reflector 120 and polarizer104, TDF can be disposed as it is in the positions of the transflectivereflector 120 and polarizer 104. In this case, similarly to theembodiment, the transmission axes of the transrucent absorber andreflection-type polarizer are aligned with the transmission axis of thefirst polarization separator 110. Moreover, when an observer viewsdisplay utilizing the light flux 406 as external light incident from theviewing side “A”, the amount of light flux 702 is reduced and therefore,when a liquid crystal panel incorporating therein a transflective layerand employed in the first embodiment is employed as the first liquidcrystal panel 502, high visibility of an image to be displayed can beachieved.

It should be noted that when a liquid crystal display device isconfigured as described above, the third polarizer 104 (absorption-typepolarizer) need not be provided on the upper side of the second liquidcrystal panel 102.

Also in those embodiments, any type of liquid crystal panel can beemployed in the liquid crystal display device and it is apparent thatirrespective of passive matrix drive or active matrix drive, any type ofdrive method for liquid crystal panel can be applied to the liquidcrystal panel. Furthermore, explanation has been made of the normallywhite mode as a display mode, the normally black mode may also beapplied to the embodiments.

Third Embodiment: FIGS. 7 and 8

Thereafter, a third embodiment of the liquid crystal display deviceaccording to the invention will be explained. FIG. 7 is a side view of alight guide plate and light source of the liquid crystal display device,and FIG. 8 is a schematic cross sectional view to explain how the liquidcrystal display device performs a display function.

The third embodiment is different from the above-described secondembodiment only in the configuration of a light guide plate. In thefirst and second embodiments, a light guide plate including prismsarranged on one surface of an acrylic plate to have a height of severalmicrometers and a constant pitch has been used. Such a light guide plategenerally needs to be manufactured with high processing accuracy. In thethird embodiment, a light guide plate that is generally said to allowfacilitated manufacture, increase in yield and simplified processing isemployed.

As shown in FIG. 7, a light guide plate 801 is made of acrylic materialhaving light-scattering capability due to beads dispersed therein andthe acrylic material is shaped into a thin wedge. Similarly to theaforementioned individual embodiments, an LED array is employed as alight source 114 and light emitted from the light source is incidentfrom one end surface 801 c positioned on the thicker side of the lightguide plate 801 in the longitudinal direction thereof. The light guideplate 801 is characterized in that while a light flux incident from thelight source is guided along the plate, the light flux is affected by adifference between refractive indexes of the beads 801 a and the acrylicmaterial, and then scattered, and finally exits to both upper and lowersides. lower sides.

The liquid crystal display device of the embodiment is configured byonly replacing the light guide plate 112 of FIG. 5 with the light guideplate 801 of FIG. 7 and therefore explanation of the configurationthereof is omitted.

How the liquid crystal display device performs a display function willbe explained with reference to FIG. 8. The reason why individual arrowsof FIG. 8 are drawn in different forms is the same as that explained inthe description of the arrows shown in FIG. 4.

The light guide plate 801 of FIG. 8 allows light from the light source114 to enter through one end surface 801 c of the plate and exit to bothupper and lower sides of the plate, as previously described, while beingguided in a direction toward the other end surface. That is, the lightflux 401 incident from the light source 114 is scattered, producinglight fluxes 401 a, 401 b, and the light flux 401 a exits in a directiontoward the polarization separator 110, and simultaneously the light flux401 b exits also in a direction toward a transflective reflector 120. Atthis point, similarly to the second embodiment, the light flux 401 aserves to illuminate the two liquid crystal panels. On the other hand,the light flux 401 b travels so that a light flux 900, a portion of thelight flux 401 b, transmits through the transflective reflector 120,serving as illumination light for the second liquid crystal panel 102. Alight flux 901 reflected by the transflective reflector 120 againreturns back to the light guide plate 801. When the light flux 901transmits through the light guide plate 801 and enters the polarizationseparator 110, the linearly polarized light component, parallel to thepolarization transmission axis of the separator, of the light flux 901transmits through the separator, serving as illumination light for thefirst liquid crystal panel 502, and the linearly polarized lightcomponent, parallel to the reflection axis of the separator, of thelight flux 901 is reflected by the separator and again transmits throughthe light guide plate 801, and is returned back to the transflectivereflector 120. While this operation is repeated, the component, parallelto the transmission axis of the polarization separator 110 or thepolarization axis of a third polarizer 104, of the light flux 901transmits through the polarization separator 110 or the third polarizer104, serving as illumination light for the first liquid crystal panel502 or the second liquid crystal panel 102.

As described above, either the light flux 401 a or 401 b exiting theupper or lower surface of the light guide plate 801 is never absorbedand both light fluxes are utilized as illumination light for the twoliquid crystal panels 502 and 102. How the display device of theembodiment operates when the light flux 406 as external light from theviewing side “A” and the light flux 404 as external light from theviewing side “B” are incident on the device is the same as thatexplained in the description of the display device of the secondembodiment and therefore explanation thereof is omitted.

It should be appreciated that the embodiment employs an acrylic materialwith inclusion of beads as a material for the light guide plate 801, buta material to be employed in the embodiment is not limited to such anacrylic material. Almost all types of generally available light guideplates can be used as they are as the light guide plate 801 of theembodiment. For example, a light guide plate formed by embossing of bothsurfaces or one surface of a transparent plate to scatter light incidentthereon or by printing of a printing material in the form of wedge tocompensate for variations in light intensity can be used as it is.

Fourth Embodiment: FIG. 9

Thereafter, a fourth embodiment of the liquid crystal display deviceaccording to the invention will be explained. FIG. 9 is a perspectiveview of the general configuration of the liquid crystal display device,omitting illustration of a polarization separator and/or a transflectivereflector.

The essential configuration of the fourth embodiment would be the sameas any one of the configurations of the first to third embodiments,however, herein is explained as having the same configuration of thefirst embodiment.

The liquid crystal display device of the embodiment is different fromthat of the first embodiment only in that the size (in this case, thewidth thereof) of a second liquid crystal panel 102 is smaller than thatof a first liquid crystal panel. Thus, the display area of the secondliquid crystal panel 102 is smaller than the display area of the firstliquid crystal panel 101.

In this case, since the second liquid crystal panel 102 is used as anauxiliary panel for the first liquid crystal panel 101 (as is the casewith the liquid crystal display device explained in the description ofthe conventional technique of FIG. 13), it could often be maderelatively small. Therefore, in the first embodiment, since the lightsource 114 is designed to illuminate the panels so that the display areaof the first liquid crystal panel having greater display area than thesecond liquid crystal panel receives a sufficient amount of light, alarge amount of light to be used to perform display on the second liquidcrystal panel having smaller display area is wasted. Since the secondliquid crystal panel 102 is used as an auxiliary panel, it also needs tobe designed to further reduce its power consumption.

In consideration of such requirement, the embodiment is configured sothat an unnecessary portion of a light source is turned off to reducewasted power.

Referring to FIG. 9, the same tranflective liquid crystal panel as thatemployed in the first embodiment is employed as a first liquid crystalpanel 101 in the embodiment. A second liquid crystal panel 102 has thesame configuration as that in the first embodiment, however has adifferent area than that in the first embodiment. That is, the majorside of the second liquid crystal panel 102 is the same as that of thefirst liquid crystal panel 101, however the short side thereof isone-third as that of the first liquid crystal panel 101, causing thesecond liquid crystal panel to have a long and narrow shape. The secondliquid crystal panel 102 serving as an auxiliary panel is comprised ofabout 10×100 dots of pixels, and is primarily used to inform a user ofcurrent time and brief information. The light source 114 disposedclosely to an end surface 112 c of the light guide plate 112 iscomprised of three light-emitting elements, i.e., Light Emitting Diodes(LEDs) 11, 12, 13 in the embodiment. Light fluxes 15 to 17 representlight fluxes emitted from the individual LEDs and entering to the lightguide plate 112.

Referring to FIG. 9, when an observer views the first liquid crystalpanel 101 from a viewing side “A”, all of the LEDs 11 to 13 of the lightsource 114 are turned on. At this point, the light guide plate 112operates so that all the light fluxes are input through the end surface112 c of the plate, guided within the entire plate, propagate along thetravel path, and exit the guide plate as explained in the description ofthe light flux 401 of FIG. 4, serving as illumination light for thefirst liquid crystal panel 101, as indicated by the light flux 402 ofFIG. 4.

Then, when an observer views the second liquid crystal panel 102 from aviewing side “B”, the LEDs 11 and 13 out of the three LEDs disposed onboth end sides of the light source 114 are turned off and only the LED12 is turned on. At this point, only the light flux 16 out of the lightfluxes 15 to 17 shown in FIG. 9 to be emitted from the individual LEDsexits the light source and enters the light guide plate 112. Within thelight guide plate 112, the light flux incident thereon propagates alongthe travel path indicated by the light flux 401 shown in FIG. 4 andexits the guide plate. Note that the light flux 401 is present onlywithin the illumination range covered by the LED 12. Referring to FIG.9, the range corresponding to one-third of the total width of the lightguide plate 112 and determined by reference to the central portion ofthe guide plate, i.e., about one-third of the whole area of the guideplate is to be illuminated. Since the light guide plate 112 is made of atransparent material, incident light travels relatively straight andtherefore a portion of the light guide plate corresponding to theone-third of the total width thereof can be uniformly illuminated.

As is already explained in the description of FIG. 4, the light flux 401is reflected by the polarization separator 110, producing the light flux403 which serves as illumination light for the second liquid crystalpanel 102. At this point, since the second liquid crystal panel 102 isdisposed in the central portion of a screen, nearly the entire area ofthe panel 102 is uniformly illuminated and therefore an observer on theviewing side “B” is able to obtain a bright display level of an image tobe displayed. In this case, since one out of the three LEDs is beingturned on, power consumption is reduced to one-third of that in the caseof turning on of all of the three LEDs.

The embodiment employs three LEDs, but is not limited to employment ofthree LEDs. The number of LEDs would be determined so that an optimalnumber of LEDs illuminate individual liquid crystal panels each havingan optional display area to allow all the panels to have desiredcharacteristics. Furthermore, the plurality of light emitting elementsare not limited to an LED, but would be a miniature lump, EL element,and other various types of light emitting elements. Even in such a case,when a liquid crystal display device is configured so that a pluralityof light emitting elements are selected from the whole of light sourceand turned on, the beneficial effects similar to those of the embodimentcan be obtained.

Moreover, it is needless to say, although the above explanation has beenmade of a liquid crystal display device configured in accordance withthe embodiment and having the configuration similar to that of the firstembodiment, even when the configuration of the embodiment is applied toa liquid crystal display device having the configuration similar tothose of the second and third embodiments, beneficial effects similar tothose of the embodiment can be obtained.

Fifth Embodiment: FIGS. 10 to 12

Thereafter, a fifth embodiment of the liquid crystal display deviceaccording to the invention will be explained.

FIG. 10 is a schematic cross sectional view, similar to that of FIG. 8,to explain how the liquid crystal display device performs a displayfunction and FIG. 11 is a perspective view of the general configurationof the liquid crystal display device, omitting illustration of twopolarization separators, and FIG. 12 is an explanatory diagramillustrating a positional relationship between the individualpolarization axes of second, third polarizers and first, secondpolarization separators of FIG. 10.

The configuration of the liquid crystal display device is almost allcommon to the configuration of the liquid crystal display device of thethird embodiment shown in FIG. 8, however the present embodiment isdifferent from the third embodiment in that instead of the transflectivereflector 120, a second polarization separator 130 having a polarizationreflective axis and polarization transmission axis similar to those ofthe polarization separator 110 is disposed between a second liquidcrystal panel 102 and a light guide plate 801. The polarizationseparator 110 is the same as those of the aforementioned individualembodiments, however it is hereinafter referred to as “firstpolarization separator” in terms of discrimination of the polarizationseparator 110 from a second polarization separator 130.

Furthermore, as shown in FIG. 11, the width of the second liquid crystalpanel 102 is reduced to about one-third of the width of the first liquidcrystal panel 502, causing the second liquid crystal panel 102 to haveabout one-third of the display area of the first liquid crystal panel502. Moreover, the embodiment includes a light source 114 comprised of aplurality of light-emitting elements, i.e., three LEDs 11 to 13 and hasthe configuration similar to that of the fourth embodiment shown in FIG.9 except that the light guide plate 112 of FIG. 9 is replaced by awedge-shaped light guide plate 801 and the first liquid crystal panel101 of FIG. 9 is replaced by a liquid crystal panel 502 having notransflective layer.

Then, an optical arrangement of a second polarizer 105, firstpolarization separator 110, and third polarizer 104 in the liquidcrystal display device of the present embodiment will be explained withreference to FIG. 12. FIG. 12 is a diagram similar to FIG. 3 and thesame signs are used to the same polarization axes as those of FIG. 3,and explanation thereof is omitted. Note that numeral 301 denotes thepolarization transmission axis of the first polarization separator 110and numeral 302 denotes the polarization reflective axis thereof.

It should be noted that a polarization transmission axis 305 of a secondpolarization separator 130 and a polarization reflection axis 306thereof orthogonal to the axis 305 are newly illustrated. As shown inFIG. 12, the second polarization separator 130 is disposed so that thepolarization transmission axis 305 and polarization reflection axis 306of the separator 130 are orthogonal to the polarization transmissionaxis 301 and polarization reflection axis of the first polarizationseparator 110, respectively.

Accordingly, a polarization transmission axis 303 of a second polarizer105 is parallel to the polarization transmission axis 301 of the firstpolarization separator 110 and a polarization transmission axis 304 of athird polarizer 104 is parallel to the polarization transmission axis305 of the second polarization separator 130. In this case, thepolarization transmission axis 303 of the second polarizer 105 and thepolarization transmission axis 304 of the third polarizer 104 areorthogonal to each other. The individual polarizers and the individualpolarization separators are disposed in accordance with theabove-described arrangement. Note that a first polarizer 103 and afourth polarizer 106 are disposed to allow the individual liquid crystalpanels 102, 502 to operate in the normally white mode.

How the embodiment shown in FIG. 10 performs a display function isdifferent from how the third embodiment explained with reference to FIG.8 performs a display function in that a light flux incident on thesecond polarization separator 130 travels so that a linearly polarizedlight component whose polarizing direction is parallel to thepolarization transmission axis 305 transmits through the secondpolarization separator 130 and a linearly polarized light componentwhose polarizing direction is parallel to the reflection axis 306 isreflected by the second polarization separator 130.

Accordingly, in FIG. 10, a light flux 401 a, a portion of the light flux401 emitted from the light source 114 and incident on the light guideplate 801, exits to the side of the first polarization separator 110 andthe polarized light component, parallel to the polarization transmissionaxis of the separator 110, of the flux 401 a transmits through theseparator 110, producing a light flux 700, and the light flux 700transmits also through the first liquid crystal panel 502 operating inthe normally white mode and exits to the viewing side “A”. Furthermore,the polarized light component, parallel to the polarization reflectionaxis of the first polarization separator 110, of the flux 401 a isreflected by the separator 110, producing a light flux 703, and thelight flux 703 transmits through the light guide plate 801 and alsothrough the second polarization separator 130 because the light flux 703is a linearly polarized light component parallel to the polarizationtransmission axis of the separator 130, and transmits also through thesecond liquid crystal panel 102 operating in the normally white mode andexits to the viewing side “B”.

A light flux 401 b, a portion of the light flux 401 entering the lightguide plate 801, exits to the side of the second polarization separator130 and is similarly divided by the second polarization separator 130into two polarized light components, a light flux 903 as a polarizedlight component parallel to the polarization transmission axis of theseparator 130 and a light flux 904 as a polarized light componentparallel to the polarization reflection axis thereof, and those lightfluxes 903, 904 exit respectively to the viewing sides “B” and “A”.

A light flux 706, a portion of the light flux 404 incident from theviewing side “B”, transmits through the second liquid crystal panel 102and the entire light flux 706 transmits through the second polarizationseparator 130, and is reflected by the first polarization separator 110and returned back, and exits to the viewing side “B”.

A light flux 701, a portion of the light flux 406 incident from theviewing side “A”, transmits through the first liquid crystal panel 502and the entire light flux 701 transmits through the first polarizationseparator 110, and is reflected by the second polarization separator 130and returned back, and exits to the viewing side “A”.

Accordingly, as is the case with the third embodiment explainedreferring to FIG. 8, the liquid crystal display device is able toperform image display by using any one of light emitted by the activatedlight source 114 and incident light from the viewing side “A” or “B”,and further provide a brighter display.

Furthermore, when a liquid crystal display device is configured so thata plurality of light emitting elements, LEDs 11 to 13, constitute thelight source 114 shown in FIG. 11 and the whole or a portion of the LEDs11 to 13 is turned on depending on the display area determined bywhether both or any one of the first and second liquid crystal panels502, 102 performs display, allowing the device to reduce its powerconsumption while performing sufficiently bright display.

Also in this case, it is needless to say, various modificationsdescribed after the explanation of the fourth embodiment can similarlybe made to the embodiment.

INDUSTRIAL APPLICABILITY

As described so far, according to the present invention, when a liquidcrystal display device includes two liquid crystal panels disposed backto back each other to enable the visual recognition of the two liquidcrystal panels, the device becomes thin and electronic equipmentsincorporating therein such liquid crystal display device increases itsportability, utilization ratio of light from backlight and reduces itspower consumption. Particularly, when a light source is comprised of aplurality of light emitting elements and the number of light emittingelements to be activated to emit light is varied depending on thedisplay area determined by whether both or any one of the two liquidcrystal panels performs display, further reduction in power consumptioncan be achieved.

The liquid crystal display device according to the invention is suitablefor use in a display unit of a portable folding phone or a displaydevice of various types of portable electronic equipments, particularly,an electronic equipment needed to perform both-sided display.

1. A liquid crystal display device, comprising: first and second liquidcrystal panels, each including liquid crystal cells having a liquidcrystal layer sandwiched between two transparent substrates, disposedback to back with respect to each other to enable visual recognition ofthe first and second liquid crystal panels; a light guide plate disposedbetween the first and second liquid crystal panels; a light sourcedisposed adjacent at least one end surface of the light guide plate; anda polarization separator disposed between the first liquid crystal paneland the light guide plate, said liquid crystal display device beingfurther configured so that light emitted from the light guide plate isdivided into two polarized lights by the polarization separator, one ofthe two polarized lights exits to a side of the first liquid crystalpanel, the other thereof exits to a side of the second liquid crystalpanel via the light guide plate, wherein the light source comprises aplurality of light emitting elements and wherein the number of lightemitting elements to be activated to emit light is allowed to differdepending on whether display is performed by the first liquid crystalpanel or the second liquid crystal panel.
 2. The liquid crystal displaydevice according to claim 1, wherein a display area of the second liquidcrystal panel is smaller than a display area of the first liquid crystalpanel.
 3. The liquid crystal display device according to claim 2,wherein when display is performed by the first liquid crystal panel, allof the plurality of light emitting elements are allowed to emit lightand when display is performed by the second liquid crystal panel, only aspecific number of the plurality of light emitting elements are allowedto emit lights to sufficiently illuminate the display area of the secondliquid crystal panel.
 4. A liquid crystal display device including firstand second liquid crystal panels, each consisting mainly of liquidcrystal cells each of which has a liquid crystal layer sandwichedbetween two transparent substrates, disposed back to back each other toenable visual recognition of the first and second liquid crystal panels,said liquid crystal display device comprising: a light guide platedisposed between the first and second liquid crystal panels; a lightsource disposed adjacent at least one end surface of the light guideplate; and a first polarization separator disposed between the firstliquid crystal panel and the light guide plate, and a secondpolarization separator disposed between the second liquid crystal paneland the light guide plate, said liquid crystal display device beingfurther configured so that lights emitted from the light guide plateeach are divided into two polarized lights by a corresponding one of thefirst and second polarization separators, one of the two polarizedlights exits to a side of the first liquid crystal panel, the otherthereof exits to a side of the second liquid crystal panel, the lightsource comprises a plurality of light emitting elements, and the numberof light emitting elements to be activated to emit light is allowed todiffer depending on whether display is performed by the first liquidcrystal panel or the second liquid crystal panel.
 5. The liquid crystaldisplay device according to claim 4, wherein the first and second liquidcrystal panels each have polarizers disposed on both sides of the liquidcrystal cell, wherein each of the first and second polarizationseparators has a polarization transmission axis for transmitting one oflinearly polarized light components whose polarizing directions areorthogonal to each other and a polarization reflection axis forreflecting the other of the linearly polarized light components, whereinthe polarization transmission axes of the first and second polarizationseparators are orthogonal to each other, wherein the first polarizationseparator is disposed so that the polarization transmission axis of thefirst polarization separator is closely aligned with a polarizationtransmission axis of the polarizer, facing the first polarizationseparator, of the first liquid crystal panel, and wherein the secondpolarization separator is disposed so that the polarization transmissionaxis of the second polarization separator is closely aligned with apolarization transmission axis of the polarizer, facing the secondpolarization separator, of the second liquid crystal panel.
 6. Theliquid crystal display device according to claim 4, wherein a displayarea of the second liquid crystal panel is smaller than a display areaof the first liquid crystal panel.
 7. The liquid crystal display deviceaccording to claim 6, wherein when display is performed by the firstliquid crystal panel, all of the plurality of light emitting elementsare allowed to emit light and when display is performed by the secondliquid crystal panel, only a specific number of the plurality of lightemitting elements are allowed to emit lights to sufficiently illuminatethe display area of the second liquid crystal panel.